1. Field of the Invention
This invention relates to methods of forming nonwoven fabrics of thermoplastic fibers by combining two or more fabric layers and ultrasonically bonding the combination. Such fabrics are normally of a heavier basis weight in the range of from 1.5 oz/yd.sup.2 (50.8 g/m.sup.2) to 9.0 oz/yd.sup.2 (305.1 g/m.sup.2), for example, and find applications requiring durability such as in recreational fabrics for tents, outer garments and similar products. These materials, when formed from thermoplastic fibers such as polyolefins, nylon, polyesters and others may be subjected to ultrasonic bonding in a patterned nip to form the desired heavier basis weight. These combined fabrics may then be further treated for water repellency or other properties depending upon the desired end use. Nonwoven materials of this nature have been marketed, for example, under the trademark EVOLUTION.RTM. by Kimberly-Clark Corporation.
For particular applications such as tenting material, for example, it is critical that the nonwoven material be free from pinholes and other defects which could cause leakage, tearing or rupture of the fabric. This necessitates strict quality control procedures and may result in certain quantities being rejected for failing to meet this requirement. In addition, the capacity to ultrasonically bond uniformly at a high rate may be restricted when forming such heavier basis weight materials. Accordingly, it is desired to improve the quality and uniformity of such nonwoven laminates and to increase rates of production.
2. Description of the Prior Art
Methods and apparatus for forming nonwoven webs by ultrasonic bonding are well known. It is also known to combine webs of nonwovens by ultrasonic bonding. For example, U.S. Pat. No. 4,311,540 to Hill dated Jan. 19, 1982, discloses the use of patterned surface means with an ultrasonic horn anvil to attain lightweight pattern fuse (thermal) bonded webs. A carrier sheet is used through the bonding nip to provide improved operating speeds and reduced horn failure. It is also known to employ a bonding medium in connection with ultrasonic bonding and to apply it to webs prior to the ultrasonic bonding nip. U.S. Pat. No. 3,660,186 to Sager and Nelson dated May 2, 1972 is an example of such an arrangement. U.S. Pat. No. 4,109,353 to Mitchell and Ostermeier dated Aug. 29, 1978 is directed to ultrasonically bonding nonwoven webs, including composite layer webs, by applying water or another liquid in sufficient amounts to provide fiber mobility at the time of application of ultrasonic energy. For example, the patent suggests that water amounts in the range of at least 500% by weight and preferably at least 1300% by weight be used. It remains, however, for particularly demanding applications such as those described above, desired to improve the quality, uniformity and production rates of ultrasonically bonded composite nonwoven web laminates.